The invention relates to a method and apparatus for providing information about the vibration of rotating blades, particularly those in gas turbine engines.
In the development of gas turbine engines, it is necessary to measure the vibration (both amplitude and frequency) of the rotating blades. From such measurements, the induced stresses in the blades may be determined and action taken to avoid stresses which are high enough to cause damage to the blades.
It is known to mount strain gauges on rotating turbine blades to provide information about the amplitudes and frequencies of vibration of the blades. One strain gauge is provided on each blade, and connected to a radio telemetry system mounted on the rotor, which transmits the measurements from the rotor. Due to the number of strain gauges required to fully determine the vibration, the telemetry system is complex, large and time consuming to install within the rotor.
An alternative technique for characterising blade rotation is xe2x80x9ctip timingxe2x80x9d in which a non-contact probe mounted on the turbine casing is used to measure the time at which a blade passes. This time is compared with the time at which the blade would have passed the probe if it had been undergoing no vibration. This is termed the xe2x80x9cexpected arrival timexe2x80x9d and can be calculated from the rotational position of the particular blade on the rotor in conjunction with a xe2x80x9conce per revolutionxe2x80x9d or xe2x80x9cOPRxe2x80x9d signal which provides information about the position of the rotor. The OPR signal is derived from the time at which an indicator on the rotor passes a reference sensor, and its use is well known in the art.
The difference between the expected arrival time and the actual arrival time can be multiplied by the turbine blade tip velocity to give the displacement of the blade from its expected position. Data from a plurality of sensors can be processed to obtain the amplitudes and frequencies of vibration of the blades.
The above tip timing method does not require a telemetry system since the probes are mounted on the casing. However, a large number of probes are required to characterise the vibration of all the blades and their installation is thus complex and time consuming.
According to the invention, there is provided a method of characterising the vibration of a plurality of rotating blades mounted on a rotor, the method including the steps of:
providing a strain gauge on one of the blades;
analysing data from the strain gauge to provide information about the frequencies of vibration and the vibrationally induced stresses in that blade;
providing at least one timing probe for detecting the times at which respective blades pass the probe;
analysing data from the or each timing probe to obtain information about the amplitudes and/or velocities of vibration of each of the plurality of blades;
determining the relationship between vibrationally induced stress as measured by the strain gauge and vibrational amplitude or velocity as measured by the timing probe(s) for the blade on which the strain gauge is mounted, for a particular frequency of vibration; and
assuming that the above relationship is applicable to each of the plurality of blades, calculating the vibrationally induced stress for each blade from its amplitude or velocity of vibration as measured by the timing probe(s) and the said relationship.
Preferably the said relationship is a ratio of vibrationally induced stress as measured by the strain gauge to vibrational amplitude or velocity as measured by the timing probe(s).
Preferably the method further includes the step of providing reference indicia on the rotor and a reference probe for providing a signal when the reference indicia passes the probe, thereby providing a signal once for every rotation of the rotor (an xe2x80x9cOPRxe2x80x9d signal).
Preferably the timing probe(s) and the reference probe are mounted on a casing surrounding the blades.
Preferably the method includes the step of determining, for each of the plurality of blades, its spatial relationship with the reference indicia on the rotor. Preferably the method includes the step of calculating an expected arrival time at which each blade tip would pass a respective timing probe were it undergoing no vibration.
Preferably the strain gauge data is analysed by Fourier analysis. The analysis for a particular frequency may include determining the amplitude of vibration of the blade and, from that information, the stresses induced in the blade by that mode of vibration.
In relation to timing probe data, if the vibration of the blades is asynchronous, the amplitude of vibration may be measured by: measuring the displacement of the blade from its expected position for each of a plurality of rotations of the blade; determining the maximum displacement of the blade from its expected position; and assuming that this maximum displacement is the vibrational amplitude. Preferably the displacement of the blade is measured for at least twelve rotations of the rotor.
Alternatively, the timing probe data may be analysed using Fourier analysis on a plurality of timing probe measurements, the excited mode or modes of vibration being indicated by peaks in the spectrum.
If the vibration of the blades is synchronous, the method may include the steps of:
determining an approximate resonance frequency for a blade;
rotating the blade at a plurality of frequencies in a range from below to above the approximate resonance frequency, and at each frequency measuring the displacement of the blade from its expected position, using a timing probe;
analysing the plurality of measured displacements at the respective plurality of frequencies to provide maximum and minimum displacement values; and
comparing the maximum and minimum displacement values to provide an indication of the amplitude of the vibration of the blade.
Preferably the method includes the use of two spaced timing probes. The probes are preferably spaced between 5xc2x0 and 20xc2x0, and typically about 10xc2x0, apart. The velocity of a blade may be measured by determining the time of flight of the blade tip between the two probes and using this time and the known physical separation of the probes to calculate a blade tip velocity. The velocity due to the steady rotation of the blades may then be subtracted to indicate the vibrational velocity.
According to the invention there is further provided apparatus for characterising the vibration of rotating blades, the apparatus including:
a plurality of blades mounted for rotation on a rotor and extending radially outwardly therefrom;
a casing at least partially surrounding the blades;
a strain gauge mounted on one of the blades;
and at least one timing probe mounted on the casing.
Preferably the apparatus further includes:
means for analysing data from the strain gauge to provide information about the frequencies of vibration of the blade and the vibrationally induced stresses in that blade;
means for analysing data from the or each timing probe to obtain information about the amplitudes and/or velocities of vibration of each of the plurality of blades; and
means for determining the relationship between vibrationally induced stress as measured by the strain gauge and vibrational amplitude or velocity as measured by the timing probe(s) for the blade on which the strain gauge is mounted, for a particular frequency of vibration; and
assuming that the above relationship is applicable to each of the plurality of blades, calculating the vibrationally induced stress for each blade from its amplitude or velocity of vibration as measured by the timing probe(s) and the said relationship.
The apparatus preferably further includes a reference probe mounted on the casing and a reference indicia on the rotor, and means for providing a signal when the reference indicia passes the reference probe.